Porous membranes are suitable for use in membrane separation of substances in a liquid wherein the substance is sieved depending on the pore size. Porous membranes are widely used in medical applications such as hemodialysis and hemofiltration as well as water treatment such as those by household water purifier and water purification.
In particular, in the field of blood purification such as hemodialysis, there is a demand for a separation membrane having a high fractionation performance capable of selectively allowing the permeation of the unnecessary low and medium molecular weight substances in the blood while not allowing the permeation of the necessary high molecular weight substance. Typical low molecular weight substances include uremic toxins such as urea, creatinine, and phosphorus. In the dialysis, these substances are mainly removed by diffusion, and therefore, the separation membrane is required to be highly water permeability. Typical medium molecular weight substances include β2-microglobulin. β2-microglobulin is a protein having a molecular weight of about 12,000, which is conceivably a substance responsible for dialysis-related amyloidosis, and accordingly, removal of this substance in the dialysis is required. On the other hand, albumin which is a protein having a molecular weight of about 66,000 fulfills the functions including maintenance of the osmotic pressure and retention and transportation of various substances. Accordingly it is a substance required to remain in the blood and its loss in the dialysis should be suppressed. Recently, it is considered that some substances with the molecular weight in the range of near 30,000 as typically represented by α1-microglobulin are also subject to the removal.
Accordingly, there is a demand for a porous membrane simultaneously exhibiting a high water permeability and a high protein fractionation performance which can be used as a separation membrane for the dialysis. In particular, in the hemodiafiltration which is a therapy recently receiving attention wherein blood which has been diluted with dialysis solution is concentrated by filtration through the separation membrane, the separation membrane used for the hemodiafiltration is required to have a high water permeability. Also required is a high protein fractionation performance so that the α1-microglobulin having a high molecular weight can be removed at a high degree while suppressing the loss of albumin.
When the pore diameter of the porous membrane is reduced to suppress the loss of albumin, water permeability will become reduced and ability of removing low molecular weight substances such as uremic toxins will also be reduced. On the other hand, when the pore diameter of the porous membrane is increased to improve the ability of removing the β2-microglobulin, amount of the albumin lost will be increased despite the improvement in the water permeability. As described above, the water permeability and the protein fractionation performance are greatly affected by the pore diameter on the surface of the porous membrane, and simultaneous realization of the water permeability and the protein fractionation performance has been difficult.
An exemplary technique for improving the water permeability and the fractionation performance of the porous membrane is the one wherein the major diameter is increased in relation to the minor diameter by stretching the surface pores. The method used for stretching the pores on the surface of the porous membrane include a method wherein the stretching is conducted after the solidification of the porous membrane and a method wherein drafting is conducted before the solidification of the porous membrane.
Patent Documents 1 and 2 disclose the porous membranes produced by the stretching. Patent Documents 3 and 4 disclose the porous membranes produced by the drafting. Patent Documents 5 and 6 disclose the porous membranes wherein stretched shape of the pores on the inner surface have been formed by adjusting the composition of the spinning dope solution and membrane forming temperature to thereby control the pore development and coagulation by the phase separation.